This invention relates to a commutatorless electronic motor having a multi-pole external rotor comprising a magnet body having north and south poles and revolving about a rotor axis, in which a multislot stator is fixed inside the external rotor, and in which at least two magnetic field sensing (magnetic-field-sensitive structural) elements are radially disposed between the magnet body and a flux return part fixed in the vicinity of the external rotor. As the rotor revolves, the magnetic field sensing elements furnish signals dependent on the angular position of the rotor to an electronic commutation device to which the signals are supplied and which is provided for current supply to the stator windings.
As electronic motor of this kind, having an external rotor, is described in the journal "Elektrodienst," volume 20 (1978), Special Issue "Standard Products," at page 11. This is a collectorless DC motor with electronic commutation which is distinguished by especially quiet running. Because the electronic motor may have a relatively low axial height, it has particular utility in flat components having direct drives, such as, for instance, video cassette recorders.
In the known electronic motor, the rotor is provided with permanent magnet poles and revolves externally about the stator. The external rotor has a hollow, cylindrical permanent magnet body surrounded by a metallic rotor cup. The magnet body may, in particular, be formed of magnetic rubber and is firmly seated against the inner wall of the rotor cup. The stationary stator consists of a cylindrical stack of laminations which is provided with slots extending obliquely to the rotor axis. The current flows through stator windings which are placed in the slots of the stator. The stator windings are energized by an electronic commutation device as a function of the rotor position in such a way that a rotary circulation results. Two magnetic field-sensing elements fixed on a support plate, namely two Hall generators, are used as position signalizers. They are mutually offset spatially and electrically in the vicinity of the end face of the rotor cut. They protrude from the support plate into an annular groove provided in the end face of the rotor cup. Thus, viewed in radial direction from the rotor axis, there results a succession in space of the permanent magnet body, a Hall generator, and the lower end of the rotor cup, which thus acts as magnetic flux return. The rotor position-dependent signals delivered by the Hall generator are supplied to the mentioned commutation device. The external rotor may have a relatively large number of poles. For instance, there may be a 12- or 16- pole external rotor and a 24- or 32- slot stator.
As mentioned before, the slots in the prior art lamination stack extend obliquely to the rotor axis. The magnet body of the external rotor has a magnetization where--seen in circumferential direction of the hollow cylindrical permanent magnet body--north and south poles alternate regularly. The neutral field lines between each north pole and the adjacent south pole are parallel to the rotor axis. The combination of slanting slots and magnetization with neutral field lines parallel to the axis has been chosen because of the requirement for an especially quiet running electronic motor.
The known commutatorless DC motor has the advantage that an axial displacement of the rotor relative to the stator has practically no influence on the magnitude of the Hall voltage of the two Hall generators. Also, the two Hall generators may be mutually displaced in the axial direction by a small distance without the need to provide for different Hall voltages. This property of the electronic motor saves special adjusting work in manufacture. On the other hand, however, the prior art motor has the disadvantage that, because of the oblique slots, a complicated winding technique must be used and the copper filling factor is relatively small.
It is an object of the invention to simplify the manufacture of an electronic motor of the above mentioned kind while providing a structure that is equally quiet in running and equally insensitive to errors in installing the magnetic field sensing elements, such as offset in the axial direction.